Growth of Icosahedral Boron-Rich Clusters at High Pressure
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BORON-RICH
CLUSTERS
AT
HIGH
PAUL F. McMILLAN, HERVE HUBERT, ANDREW CHIZMESHYA Materials Research Science and Engineering Center, Dept. of Chemistry and Biochemistry and Center for Solid State Science, Arizona State University, Tempe, AZ 85287 [email protected] ABSTRACT The stoichiometry of boron suboxide (B6 O 1-x) synthesized at high pressure lies closer to the nominal composition (x = 0) than materials obtained at atmospheric pressure. The crystallinity of materials obtained in the presence of molten B20 3 is also higher than for sintered powders. Further, for syntheses at 1700-1800 °C between 4 and approximately 56 GPa, the well-crystallized particles are dominated by large (up to --40 R&m in diameter) icosahedral multiply-twinned particles. This unusual morphology is obtained by Mackay packing; i.e., by assembly of successive shells of icosahedral B12 units around a central icosahedral nucleus. The result is a multiply-twinned particle in which each of the 20 elements has theR3m space group of the a-B structure. INTRODUCTION Boron-rich solids are good candidates for "super-hard" materials, with high tensile strength and large values of the bulk modulus and cohesive energy. There have been reports that some boron suboxide materials ("B 22 0") have scratched faces of diamond [I], and the "true" boron suboxide phase B6OIx has an indentation microhardness reported at 38 GPa [21. There have been several previous reports of "B 6 0"
synthesis at room pressure formed by
reaction of elemental B with B20 3 or by oxidation of boron with zinc oxide 12-61. These syntheses typically produce fine-grained powders mixed with amorphous (a-B) material. Near-fully dense material has also been obtained by hot-pressing [2,3,71. When analyzed, these "B 6 0" materials are nonstoichiometric with reported compositions ranging from B6 0 0.72 to B6 0 0 .86 [3,8-10]. Although no single crystals have been available for structure refinement, the crystal structure has been determined from powder data. The a-boron structure (space group R3m) consists of eight B 12 icosahedra situated at the vertices of a rhombohedral unit cell, and can be described as a distorted cubic close packing (ccp) of Fig. 1. SEM micrograph of a B,0 the icosahedral units. In B6 0, two 0 atoms are icosahedral multiply twinned particle located in the interstices along the [111] (ref 11-13). This particle is rhombohedral direction, replacing the 3-center, 2- approximately 5 t•m across. Much electron bond that links the icosahedra in this larger particles (up to -40 pn) are direction in the a-B structure [6,101. obtained in syntheses at 4-6 GPa. We have recently reported synthesis of "B 6 0" using high pressure-high temperature synthesis methods [11-131. These syntheses yielded materials with compositions close to
453 Mat. Res. Soc. Symp. Proc. Vol. 499 ©1998 Materials Research Society
pressure syntheses. nominal (up to B600.96), Large (up and to with 40 Ixm) mucheuhedral higher crystallinity grains were than recovered obtained in some for the runs. room In additio
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